Up to now, a number of technologies relating to exhaust gas purification have been proposed. In particular, there have been many proposals relating to treatment technologies for exhaust gases generated from internal combustion engines such as gasoline engine, diesel engine, and an engine using a bio-based fuel or a fuel including methanol.
As a catalyst for purifying an exhaust gas generated from internal combustion engine, a three-way catalyst which removes concurrently nitrogen oxide (hereinafter, also referred to as “NOx”), carbon monoxide (hereinafter, also referred to as “CO”), and hydrocarbon (hereinafter, also referred to as “HC”), a catalyst which removes concurrently NOx, CO, and HC in an oxygen-excess state due to combustion of fuel-lean, and the like have been proposed. That is, these are a type of catalysts having functions to oxidize CO and HC to CO2 and reduce NOx to N2 together. Many catalysts of this type have an Oxygen Storage Capacity (hereinafter, also referred to as “OSC”) with which catalyst itself accumulates oxygen when exhaust gas is in an oxygen-excess state (oxidative atmosphere), and releases the accumulated oxygen when exhaust gas is oxygen-deficient state (reductive atmosphere). A promoter component having such OSC is referred to as oxygen storage material (hereinafter, also referred to as “OSC material”), and by using this, CO and HC can be efficiently oxidized to CO2 even in an oxygen-deficient state. As an OSC material, it has been known that cerium oxide (CeO2: also referred to as “ceria”) and ceria-zirconia complex oxide (CeO2—ZrO2 complex oxide) have superior performance.
In order to improve performance and durability of the OSC material, a technology has been developed where a metal element such as iron is added to CeO2, CeO2—ZrO2 complex oxide, and the like.
For example, Patent Literature 1 discloses an OSC material comprising besides zirconium, cerium, and a rare earth metal as a stabilizer, 0.01 to 0.25 mol % of at least one kind of metal selected from a group consisting of iron, copper, cobalt, nickel, silver, manganese, and bismuth. And, the literature describes that these metals exist as a solid solution in a crystal structure of the OSC material.
In addition, Patent Literature 2 discloses an OSC material comprising a carrier containing ceria (CeO2) andiron oxide as an active species contained in said carrier. And the literature describes that the carrier containing CeO2 is preferably a solid solution of CeO2—ZrO2. Further, the literature describes that content of iron oxide is desirably in a range of 2 to 30% by weight as Fe2O3 relative to the weight of the OSC material, and substantial OSC cannot be obtained when the content deviates from this range.
The OSC material containing cerium functions as a promoter of oxidation/reduction reaction, and in the technologies described in the above Patent Literatures 1 and 2, iron is allowed to exist in the vicinity of cerium for the purpose of facilitating this catalytic performance of cerium.
In addition, Patent Literature 3 discloses a catalyst for purifying exhaust gas where iron oxide, cerium oxide, and at least 2 kinds of noble metal elements are supported on a refractory carrier mainly composed of alumina (Al2O3). Said catalyst is characterized in that the catalyst components are supported on said refractory carrier by using aqueous solution A containing at least one kind of noble metal element compound, organic acid cerium salt, and water-soluble iron salt, and aqueous solution B containing other noble metal element compound, as an impregnating solution.
Further, Non-Patent Literature 1 discloses that in a three-way catalyst where 1% by mass of Pd—Rh is supported on 20% by mass of CeO2—Al2O3, OSC is improved by adding 0.1 to 0.3% by mass of iron.